Four additional healthcare workers in Missouri who were in contact with a bird flu patient developed mild respiratory symptoms but the virus was not confirmed in any of them, U.S. health officials said on Friday.

So far, six healthcare workers who came in contact with the Missouri patient have developed symptoms. Only one of them was tested negative for the virus via a PCR test, while the blood samples of the others have been sent for antibody testing, the Centers for Disease Control and Prevention said.

Unlike the previous U.S. bird flu cases this year, the Missouri patient had no known contact with infected animals, raising concerns the virus may have mutated in a way that makes it spread more easily in people.

"These cases underscore the need to take this outbreak more seriously than it has been taken," said Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security.

Results from the antibody testing need to be promptly obtained to assess the risk of human-to-human spread of the virus, Adalja said.

The Missouri case was the 14th person in the United States to be diagnosed with bird flu this year. The other 13 cases were among farm workers and linked to outbreaks on poultry or dairy farms.

Missouri is leading the state's bird flu investigation with remote assistance from the CDC. (Reporting by Mariam Sunny, Kashish Tandon in Bengaluru and Julie Steenhuysen in Chicago; Editing by Maju Samuel and Krishna Chandra Eluri)